PITTSBURGH—If the world ever tries to offset global warming by reflecting more sunlight back into space, one size won't fit all. At least that is the consensus by Carnegie Mellon and Oxford University researchers who studied more than 50 scenarios for using solar radiation management (SRM) to alter climate.

The idea behind SRM is to place very fine particles in the stratosphere to reflect a small amount of sunlight and cool the earth. Every few decades nature does this after large volcanic eruptions. Because the world has made no progress in reducing the emissions of carbon dioxide that causes warming, recently more and more scientists have been looking to SRM as a possible emergency back-up plan.

But in a July 18 research paper posted on the Nature Geoscience website, Carnegie Mellon's Katherine Ricke and M. Granger Morgan and Myles R. Allen of the University of Oxford, report that while SRM can lower temperatures, the amount that is best for one part of the world will not be the same for others. "For example, we found that the best interventions for India and China are different, and that those differences increase as time goes by and more and more carbon dioxide enters the atmosphere," according to Ricke, a Ph.D. student in the Department of Engineering and Public Policy at Carnegie Mellon.

When most people first hear about SRM their reaction is "messing with planet earth is a terrible idea," said Morgan, a university professor and head of Carnegie Mellon's Department of Engineering and Public Policy. "We don't want to see this happen either," Morgan said. "But doing SRM is likely to be cheap, so there is risk that a single nation or region might start doing it to solve a local or regional climate problem, and impose the impacts on all of us. The other possibility is that the world might face a climate emergency, and find we collectively need some SRM because a billion people are at risk."

Morgan argues that "for both these reasons we need research now to better understand whether and how SRM might work, what it would cost, and what risks and nasty surprises it might carry."

Researchers already know about one of the risks. While SRM can cool the planet, it would do nothing to stop the rise of carbon dioxide, much of which ends up in the oceans, making them ever more acidic.

"If the world doesn't get serious about achieving a dramatic reduction in emissions of carbon dioxide, the planet will have lost most of its coral reefs by the end of this century along with the fish and other marine life that they depend on," said Morgan. "We need to understand SRM," he said. "But it is no substitute for finding ways to reduce carbon emissions by 80 percent as soon as possible."